Most existing offshore oil and gas fields are drilled and produced from rigid structures which rest on the ocean bottom and extend upward to a work deck situated above the ocean surface. A key design constraint for such structures concerns limiting the dynamic amplification of the structure's response to waves. Failure to minimize such dynamic amplification will diminish the fatigue life of the structure, and in extreme cases can impose excessive loadings on key components of the structure. Avoidance of dynamic amplification is typically achieved by designing the structure to have rigidity sufficient to ensure that all of its natural vibrational periods are less than the shortest period of significant energy waves to which the structure will be exposed. For most offshore locations the shortest significant wave period is about seven seconds.
This type of structure, commonly termed a "rigid platform" or "fixed platform", has proved very satisfactory for applications in up to about 300 meters of water. However, as water depths exceed this, maintaining the fundamental natural vibrational period below seven seconds requires rapidly escalating stiffness. As a result, the cost of a rigid platform begins to increase rapidly as a function of water depth in depths beyond 300 meters.
For deep water applications, it has been proposed to depart from conventional rigid structure design and develop platforms having a fundamental natural period greater than the range of periods of ocean waves containing significant energy. Such platforms, termed "compliant structures," do not rigidly resist waves and other environmental forces, but instead compliantly resist environmental loads, undergoing significant lateral motion at the ocean surface either through sway (pivoting of the structure about its base) or bending (flexure of the structure about its length). The use of a compliant offshore structure effectively removes the upper bound on the sway or bending period, thus avoiding the most troublesome design constraint of rigid structures. This greatly reduces the increase in the volume of structural material, and hence cost, required for a given increase in water depth.
Because economic considerations have not yet warranted extensive exploitation of offshore hydrocarbon reserves in water depths greater than about 300 meters, the development of compliant structure technology is currently at a fairly early stage. However, several types of compliant structures have been designed and a few have been constructed. One of the most promising concepts for achieving compliancy is incorporated in a proposed structure known as the compliant piled tower. The compliant piled tower is a slender, substantially rigid space-frame tower extending from the ocean floor to a position above the ocean surface, where it supports a deck. The tower is not rigidly tied to the ocean floor, as is a conventional platform, but rather is permitted to tilt about its base. This permits the structure to respond compliantly to waves, wind and currents. The sway of the tower is stabilized by piles which extend upward from positions surrounding the base to a pile attachment position located a preselected elevation above the ocean floor. In response to sway of the tower away from the vertical, the piles establish a righting moment acting at the point of pile attachment. This provides the stabilization necessary to restore the tower to a vertical orientation. One type of a compliant piled tower is detailed in U.S. patent application Ser. No. 806,055, filed Dec. 5, 1985 and assigned to the assignee of the present application.
A key problem in developing a practical compliant piled tower centers on the design of the stabilizing piles. Initial conceptual designs for the compliant pile tower proposed the use of tubular members having a constant wall thickness and diameter. This does not, however, accommodate the competing requirements of those sections of the pile above and below the ocean floor. The section of the pile below the ocean floor should have a relatively large diameter and large wall thickness to satisfy driving and foundation considerations. However, that portion of the pile extending upward from the ocean floor to the attachment point on the tower should have a smaller diameter and wall thickness to yield the necessary longitudinal flexibility and to present the smallest possible cross-section to ocean currents.
It would be desirable to develop a pile assembly for a compliant piled tower which satisfies these competing pile requirements while permitting a simple and quick pile driving and attachment procedure in the course of platform installation.